Transcription of Chirality in the Organogel Systems Dictates the Enantiodifferentiating Photodimerization of Substituted Anthracene

摘要

An organogelator (G) that contains 2-anthracenecarboxylic acid (2Ac) attached covalently to a gelator counterpart that consists of 3,4,5-tris(n-dodecan-1-yloxy)benzoic acid by means of a chiral amino alcohol linkage has been synthesized. G acts as an efficient gelator of organic solvents, including mixed solvents and chiral solvents. Photodimers isolated after the photoreaction of the gel samples display different degrees of stereoselectivity. In the gel state, the formation of head-to-head (h-h) photodimers is always favored over head-to-tail (h-t) photodimers. Enantiomeric excess (ee) values of the major h-h photodimers reached as high as -56% in the case of the gels with enantiomeric glycidyl methyl ethers. Here, the solvent chirality is outweighed by the intrinsic chirality of the gelator molecule. The packing of the chromophore in the gel state has been characterized by the absorption and the emission behaviors and their variations during, the course of gel-to-sol phase transition. Whereas for the hexane gel, emission intensity increases with an increase in temperature, other systems show a decrease in emission intensity. Redshift of the lambda(max) in the gel spectra indicates the J-aggregate arrangement of the chromophores. Chiral transcription in the gel state has been investigated by CD spectroscopy, which shows a decrease in CD intensity during the gel-to-sol phase transition. The X-ray diffraction study clearly differentiates among the gels in terms of the order of molecular arrangements. The gel systems are categorized as strong, moderately strong, and weak, that originate from the cooperative or individual participations of in hydrogen-bonding and pi-pi interactions, fine-tuned by the solvent polarity and the gelation temperature. A simple model based on the experimental findings and the molecular preorientation as evidenced by the stereochemistry of the photodimers has been proposed.